External features shape, size and colour: Most sea stars possess a pentamerous radial symmetry. Body consists of an indistinct central disc and five radiating and elongated rays or arms. Size averages from 10.
0 to 20.0 cm in diameter with variable colours like shades of brown, yellow, orange, pink and purple. Body is strongly flattened with distinct oral and aboral surface. Oral surface : It is the ventral surface of the central disc which is directed to the substratum.
On this oral surface, the mouth or actinosome is found at the center. Mouth is a pentagonal aperture, each angle directed towards an arm. It is surrounded by a soft perioral membrane or peristome and is guarded by five groups of oral spines or mouth papillae. Along each arm, a narrow groove called ambulacral groove is guarded on each side by two or three rows of movable calcareous, ambulacral spines. They are capable of closing over the groove.
Along the rows of these spines 3 rows of immovable spines are also found. Further, two double rows of short, tubular retractile tube feet or podia are found in each ambulacral groove. Tube feet are with suckers and serve for locomotion, capturing the food and respiration etc.
Tip of each arm bears terminal tentacle, serves as a tactile and olfactory organ. At its base a bright red eye spot made up of several ocelli is found. Aboral surface : It is the upper and slightly convex dorsal surface of the body. Along the axes of arms, a large number of short immovable, calcareous tubercles are arranged in irregular rows. Around and in between these spines occur tiny pincer-like structures, the pedicellariae. These are grasping organs used for cleaning or protecting the body surface.
They also occur on oral surface. On both the surfaces, minute finger-like, hollow and retractile processes, the dermal branchiae or gills or papulae are found. They open outside through dermal pores and serve for respiration as well as for excretion.
Near the center anus is situated. In an inter-radius between two arms madreporite, a sieve like plate with numerous narrow and radiating grooves bearing pores are situated. Madreporite leads into the water vascular system.